Estimation of lightning shielding failures and mid-span back-flashovers based on the performance of EHV double circuit transmission lines

In Japan, many unbalanced installation double-circuit EHV transmission lines were constructed to prevent simultaneous lightning faults on both the circuits. A one-line fault on the higher insulated circuit is either a shielding failure or a mid-span back-flashover. To discriminate shielding failures and mid-span back-flashovers among all the faults, the author analyzed the lightning performance of these unbalanced insulation lines. Both the unbalanced insulation lines and the balanced insulation lines are similar in the probability of causing a shielding failure and a midspan back-flashover, since both are the same in the configuration of shielding wires and phase conductors independent of the insulation mode. As a result of analysis, the author estimated that about 25~40% of all the lightning faults on the double-circuit balanced insulation lines are shielding failures, and that few mid-span back-flashovers happen there